METABOLIC PROCESSES 271 



organs by which it is fed and the wear and tear of its machinery 

 continually made good, as it is nourished and sustained a 

 living organism. 



The combined processes of up-building and breaking down in 

 the living body constitute metabolism. A well-equipped labora- 

 tory presents facilities for illustrating the actual chemical changes 

 which take place, but only in the living cell are they metabolic. 



Attention has been directed to the four classes of substances 

 composing the body tissues, and the corresponding food-substances 

 supplied to them in the dietary. We have seen that these foods 

 are presented to the body in compounds more or less complex (most 

 of them insoluble) and we have traced (briefly) their changes in the 

 system from the time of ingestion to their disposition in the 

 tissues or their expulsion as excreta. It remains to review them 

 from the standpoint of their value in metabolism. 



All foods must subserve one or more of three purposes, namely: 

 to evolve heat; to release energy ; to repair waste. 



Proteins, being reduced by digestion to simpler forms (peptones 

 and amino-acids) are absorbed and appear reconstructed in muscles, 

 blood, lymph and milk, mainly (in all tissues actually), where by 

 their nitrogenous portion they contribute to the " betterment of 

 cell conditions'' and compensate for wear and tear everywhere in 

 the body; by their non-nitrogenous elements they evolve heat 

 and create energy. 



Protein wastes (or excreta) take the form of urea uric acid and 

 compounds ammonia excreted through the kidneys. 



Carbohydrates, taken as sugar or converted into that form, 

 are absorbed and circulated, stored in liver and muscle as glycogen 

 to appear when needed as sugar again to be oxidized for the pro- 

 duction of heat and energy or stored as fat. Their excreta are CO 2 

 and water. 



Fats, after digestion, are distributed for use in many parts, 

 their oxidations evolving heat and motion, or are stored in sub- 

 cutaneous fascia, marrow, between viscera, etc. Their excreta 

 are CO2 and water. 



Mineral Salts and water are absorbed together in solution and 

 distributed to aid in the formation of various tissues throughout 

 the body. 



It thus appears that all foods contribute to body heat and 



